Hydrocarbon fluid pipelines which operate at elevated pressures can become occluded by inclusion complexes in the form of clathrates and, more specifically, in the form of solid-phase gas hydrates. Gas hydrate formation is a well known phenomenon which occurs when free liquid water is present in a hydrocarbon fluid stream flowing through a pipeline and the temperature of the hydrocarbon fluid stream falls below a critical value. The critical temperature value for gas hydrate formation can be in excess of 60.degree. F. in higher pressure pipelines, although the critical temperature value for gas hydrate formation generally decreases as a function of decreasing pressure in the pipeline.
A number of techniques are known in the prior art for preventing or inhibiting gas hydrate formation in hydrocarbon fluid pipelines. One method is to dehydrate the hydrocarbon fluid stream before transporting the hydrocarbon fluid through the pipeline. Another method is to continuously inject a gas hydrate liquid inhibitor composition, such as methanol or glycol, directly into the pipeline where the inhibitor mixes with the flowing hydrocarbon fluid. Yet another method is to wrap the pipeline wall with an electric resistance, heat-tracing cable or a heat-tracing tube containing a circulating hot fluid which elevates the temperature of the pipeline wall. A more recently developed method is to induction heat the pipeline wall using an alternating current. This method requires an electrically isolated parallel conductor in addition to the pipeline itself, such as a second outer metal pipeline surrounding, but insulated from, the original pipeline or a conductor cable running parallel to, but likewise insulated from, the original pipeline.
The above-recited prior art methods for preventing or inhibiting gas hydrate formation are excessively expensive or complex for many off-shore and subsea applications. For example, the prior art method of continuously injecting a gas hydrate liquid inhibitor, such as methanol, into a subsea production pipeline requires the operator to run a separate methanol injection tube to the subsea well head. In addition large quantities of methanol must be handled by the operator and are consumed while practicing the method. The prior art method of induction heating the pipeline wall with an alternating current requires reliable electrical isolation between the original pipeline and the parallel conductor which is extremely difficult to achieve in practice. A short circuit current path could render a large portion of the induction heating system inoperative.
The present invention recognizes a need for a more cost effective and less complex system for preventing or inhibiting gas hydrate formation in hydrocarbon fluid pipelines. Accordingly, it is an object of the present invention to provide a system for inhibiting the formation of gas hydrate blockages in a hydrocarbon fluid pipeline where a liquid water phase is coexistent with one or more hydrocarbon phases. It is another object of the present invention to provide a system for removing gas hydrate blockages in a hydrocarbon fluid pipeline which have formed as the result of unusual pipeline conditions or upsets. It is still another object of the present invention to provide such a system which can be designed as a new installation or which can be readily retrofitted to an existing system. It is yet another object of the present invention to provide such a system which has particular utility to offshore deep water hydrocarbon pipeline applications and to sub sea oil and gas well production flow line applications. These objects and others are achieved in accordance with the invention described hereafter.